1. Field
The following description relates to an apparatus and method for compensating for an artifact of higher order diffusion Magnetic Resonance Imaging (MRI).
2. Description of Related Art
The term ‘diffusion’ is interchangeable with the term ‘spreading.’ When imaging a diffusion of water molecules in vivo, such as in a brain, use of a MRI scheme is referred to as ‘diffusion MRI.’ Generally, diffusion refers to an isotropic diffusion in which molecules move randomly without a predetermined orientation, as if a drop of color ink spreads on a tissue. However, in a space filled with nerve bundles, such as a brain tissue, diffusion may occur based on a structure of the nerve bundles. Additionally, water molecules may be diffused in spite of being restricted in a part of tissues having an abnormality, such as a lesion.
A Diffusion-Weighted Imaging (DWI) pulse sequence has been developed to be applied to a diagnosis using a structural characteristic of the brain tissue and a diffusion of water molecules in the brain tissue.
A large number of spins exist in a single volume pixel (voxel). When a strong magnetic diffusion gradient is applied, a phase of a material that is well diffused (namely, well moved) in the single voxel may be severely shifted. Accordingly, phases of the spins may be dispersed, which may result in a reduction in signal.
Conversely, in a material that is difficult to be diffused, phases of spins may not be shifted in a strong magnetic diffusion gradient, and, accordingly, a signal reduction may be virtually eliminated. The level of the signal reduction may be influenced by a diffusion level of a tissue, and strength of a magnetic diffusion gradient. To maximize a weak signal reduction due to diffusion, a very strong magnetic diffusion gradient needs to be additionally used. As the strength of the magnetic diffusion gradient is increased, an image in which diffusion is emphasized may be obtained.
DWI is currently used in the form of a Magnetic Resonance (MR) image and an Apparent Diffusion Coefficient (ADC) map to diagnose diseases, such as brain tumors, cerebrovascular diseases, and the like. DWI-related technologies are being developed and applied to Diffusion Tensor Imaging (DTI) and a High-Angular-Resolution Diffusion Imaging (HARDI) method. The DTI refers to a method of applying different directions of a diffusion gradient, obtaining a diffusion direction of a voxel from a tensor model, and reconstructing a tissue structure. The HARDI method, an example of which is a q-Space Imaging (QSI) method, may improve a diffusion directional resolution by applying a significantly larger number of directions of a diffusion gradient than in the DTI.
The DTI visualizes a structure of a neuronal fiber bundle of a white matter in a deep brain. The DTI, as a clinical application, may use a diffusion encoding gradient in the range of 6 to 30 directions, or occasionally in 60 directions. However, considering a current level of angular discrimination ability, there is a limitation to discriminate a crossing fiber and a kissing fiber.
Accordingly, recently, to improve a diffusion angular resolution, q-ball imaging, Diffusion Spectrum Imaging (DSI), and the like are being developed. Such HARDI methods use at least hundreds of directions of a diffusion encoding gradient, and, accordingly, have a disadvantage of a long scan time. However, a scan time may be achieved within 1 hour, based on the above functional advantage, such as high angular resolution, protocol optimization, and the like.
For example, a DSI method needs to apply approximately 203 directions of a magnetic diffusion gradient. Accordingly, a very long scan time may be required, and thermal noise may occur due to heat generation using a strong magnetic diffusion gradient beyond a range of 4000 s/mm2 to 6000 s/mm2. Additionally, since a strong magnetic diffusion gradient is applied based on various angles within a short time, an eddy current artifact may be generated.
Additionally, a region to be scanned may move due to, for example, breathing or a heart beating. For example, during imaging of a head, any motion may have a very pronounced influence on diffusion tracking.
FIG. 1 illustrates a DSI-203 scan protocol 100 showing an analyzed motion pattern of an image that is actually obtained by DSI. FIG. 2 illustrates a shift pattern 200 of raw timecourse data in association with the DSI-203 scan protocol 100 of FIG. 1.
In scanning using the DSI of the related art, Diffusion-Weighted images need to be acquired in different directions, namely, in approximately 203 orientations. Accordingly, a scan time may be lengthened. Typically, a scan time of about 1 to 2 hours may be required.
Additionally, in the scanning using the DSI of the related art, an image quality may be reduced, and an image may be shifted, due to use of a strong diffusion gradient field.
Furthermore, in the scanning using the DSI of the related art, an image drift motion may occur due to an increase in temperature caused by scanning for a long period, and an image fluctuation motion may occur due to the strong magnetic diffusion gradient.